Ultrahigh loading dry-process for solvent-free lithium-ion battery electrode fabrication

The current lithium-ion battery (LIB) electrode fabrication process relies heavily on the wet coating process, which uses the environmentally harmful and toxic N-methyl-2-pyrrolidone (NMP) solvent. In addition to being unsustainable, the use of this expensive organic solvent substantially increases the cost of battery production, as it needs to be dried and recycled throughout the manufacturing process. Herein, we report an industrially viable and sustainable dry press-coating process that uses the combination of multiwalled carbon nanotubes (MWNTs) and polyvinylidene fluoride (PVDF) as a dry powder composite and etched Al foil as a current collector. Notably, the mechanical strength and performance of the fabricated LiNi0.7Co0.1Mn0.2O2 (NCM712) dry press-coated electrodes (DPCEs) far exceed those of conventional slurry-coated electrodes (SCEs) and give rise to high loading (100 mg cm−2, 17.6 mAh cm−2) with impressive specific energy and volumetric energy density of 360 Wh kg−1 and 701 Wh L−1, respectively.


Reviewer #1
This work reports a dry press-coating technique to fabricate the high loading electrode for lithium-ion batteries. The authors use MWNT / PVDF composite scaffold as the active material host and an etched Al foil as a current collector. With the mixing, casting, hot press, and roll process, the thick electrode with strong internal cohesion was achieved. Relatively high electrode loading (71 mg cm -2 , 13.2 mAh cm -2 ) was also realized in this work. The general impression after reading this work is that it's a good report on the evaluation of process parameters. But the concept and mechanism discussion cannot meet the quality requirement for Nat Commun.
For dry process electrode in the application of LIBs, MWNT and PVDF are common additives for conducting and binder purpose. The process used in this work, including the powder compression, hot press, and roll to roll process have all been widely reported in this field. I don't see significant improvement in this aspect.
The authors also compared the morphology difference between the dry process electrode and slurry coating ones. It's normal that the loading of active materials in dry process is much higher than slurry process, but it doesn't mean electrode in dry process can outperform the slurry process.
In the performance evaluation, the cycle performance of the slurry process electrode drops 70% in only 400 cycles. This doesn't represent the true performance of normal SCEs and is misleading. The authors also present the pouch cell results and give the energy density of the pouch cell. I suggest the authors remove the energy density data at 370 Wh/kg, the weight need to be derived from the whole cell mass that weighted of the accrual cell instead of estimation. Otherwise, the energy density result is also misleading.
→ The authors would like to first appreciate the reviewer for raising the concerns, which we believe is caused by misunderstandings of the core propositions of our manuscript. Please thus allow us to clarify. The main point of our work is to demonstrate that with the proposed dry presscoating technique, a breakthrough performance and brand-new manufacturing concept of the dry LIB electrode can be achieved, that is, a scalable ultrahigh electrode loading (71 mg cm -2 , 13.2 mAh cm -2 ) lithium metal battery with a superb energy density of 763 Wh L -1 . The dry MWNT-PVDF composite uniquely forms a robust and uniform coating layer on the etched Al current collector with a simple hot-pressing method, which is a previously unseen phenomena of MWNT in a solvent-free medium. The solid-solid adhesion mechanism introduced in other dry LIB electrode reports necessitate the help of adhesion inducing agents or techniques such as the holey graphene (HG), paraffin wax, electrostatic spray deposition (ESD), spray drying technique, gravure printing method, etc. However, the use of such adhesion inducing agents or techniques pose various limitations like a low mass loading, weak cohesion, delamination upon bending, negligible flexibility, scalability issue, etc. The ultrahigh loading full operational DPCE distinguishes it from all previous dry processed electrodes that show a loading threshold way below the standard we set here, for example, with no more than 70 mg cm -2 mass loading or only 10 mAh cm -2 areal capacity (Advanced Materials Technologies 2.10 (2017): 1700106). We have recently achieved a new record of 92 mg cm -2 mass loading DPCE with a remarkable areal capacity of 16 mAh cm -2 . A comparison with recent refs [1][2][3][4][5][6] is shown in the Another point to emphasize is the fact that unlike other dry processed electrode papers, the demonstration of the ultrahigh loading DPCE highlighted at the end of the result and discussion section was not contrived by sacrificing the scalability or performance aspect of DPCE. Normally, there exists a trade-off relationship between the electrode mass loading and the adhesion with the current collector because of the maximum threshold of adhesive force at the interlayer as well as the cohesive force between the electrode particles. Furthermore, the increase in the mass loading give rise to a thick and bulky electrode layer with a restricted tortuosity and high cell resistance which negatively affect the electrochemical performance of the cell. Whereas the DPCE presented in our manuscript does not have the aforementioned issues but in turn shows stronger adhesion with the current collector as the loading increases and exhibits a smooth cycling performance at a pouch-cell level. Thus, we have not only secured the mechanical property but also the electrochemical property of the DPCE which can aid towards downsizing battery packs in electric vehicles.
The reviewer mentioned that the higher loading of active materials in the dry process compared to slurry process is normal, and also raised the concern that the aforementioned factor doesn't mean that electrodes in the dry process can outperform the slurry processed ones. We would like to emphasize first that our manuscript was built upon the electrochemical performance data of DPCEs, which proves its superiority over the SCEs on various parameters such as the discharge capacity under variable current densities, a long-term cycle stability, impedance, Li-ion diffusion coefficient etc. To level the playing field, the mass loading as well as electrode composition of both DPCEs and SCEs were fixed and examined with at least 3 sample cells each. It was noteworthy that the DPCEs excelled in every electrochemical analysis compared to SCEs and achieved a higher capacity retention result even at a full-cell level (84 vs. 78%). Furthermore, the dry press-coating technique allowed the fabrication of ultrahigh loading electrodes (here it is ≥ 30 mg cm -2 ) and led to breakthrough performances, which was definitely not possible to achieve with the conventional slurry-coating process. Therefore, we can say that DPCEs can set a new level of competing ground which is beyond reach with the conventional solvent processed electrodes.
Additionally, the reviewer mentioned that the dropping of cycle performance of the slurry process electrode to 70% in only 400 cycles doesn't represent the true performance of normal  The comparison with other reference electrodes clearly shows that the cycle performance of SCE is not an underrated result but rather a challenging data for the juxtaposition, as it outperforms all other reference electrodes even at a much higher mass loading. Although, an exact one-to-one comparison against SCE condition could not be done due to a lack of prior example, but we tried to find as many reference electrode data as possible from high impact journals, even including those that were tested under similar but milder conditions to verify our result. As mentioned earlier the performance data of SCE was extracted from the best outcome of a group of replicate sample cells, and it was done to accurately depict the DPCE's performance and more so not to overemphasize its true electrochemical properties. Therefore, we firmly believe that our SCE's cycle performance is a reasonable parameter for the comparative analysis.
In regard to the comment that the energy density of the pouch cell needs to be recalculated based on the whole cell mass, the authors genuinely agree with the reviewer and also feel the needs to make amendments as advised. However, the reason why we excluded the mass of the packaging materials of the pouch cell in the first place was because many previous works on high loading electrodes calculated the energy densities without considering the cell package. Below are some of the captured images from the previously reported works:  lithium-ion batteries". The specific energy density was calculated without including the coin cell packaging materials.

Figure.
A captured image from a research article published in Nat. Energy, 6, 495-505, (2021), "Ultra-high-voltage Ni-rich layered cathodes in practical Li metal batteries enabled by a sulfonamide-based electrolyte". The specific energy density was calculated without including the pouch cell packaging materials.
Note that the energy densities were calculated based on just the inner materials, without including the packaging components. Therefore, to make a fair comparison with those of previously reported high-loading electrodes, the energy densities of the DPCE pouch cell were calculated without including the packaging materials.
However, the more accurate expression of energy density is unarguably the one that includes the entire mass of cell. Also, we believe that such representation can give our result an edge over others. Therefore, we recalculated the specific energy and volumetric energy density values based on the weight and volume of the accrual cell (including the cell package) and provided the information in Fig. 6e

Reviewer #2
This manuscript reported fabrication of thick electrodes with dry press process and showed promising results. Dry processing poses significant benefits in reducing manufacturing cost and environmental impact. This is an interesting area to explore. However, the work was not well presented and can't be published in current version. Below are some comments: "As a breakthrough approach, the dry process is considered a new electrode fabrication method for post-LIB electrodes because they offer unparalleled advantages in terms of operating cost and energy efficiency compared to the conventional solvent process. Moreover, the dry process can pave a path to battery miniaturization as the absence of solvent elevates the maximum threshold of active mass loading and thus allowing the fabrication of higher mass-loading electrodes 14-18 ." 18.
2) The motivation of using MWNT should be provided in the introduction. The sentences in page 6 line 103 is better suited for introduction.
→ Our deep appreciation is devoted to the reviewer's insightful comment. We also agree that the given guidance can greatly improve the logical flow of our presentation. In response to the reviewer's comment, some content restructurings were made and the motivation of using MWNT was transferred to the introduction. Below is the revised manuscript:

[Revised manuscript]
"These previous reports on the dry LIB electrode process have mainly focused on either changing the coating process or the binder to increase the performance of the dry electrode but rarely explored alternative ways, such as employing a new conductive agent or current collector, to answer the core challenges of solvent-free electrode fabrication, which include weak cohesive strength, low deformability, high cell polarization, low rate capability, etc.
Carbon nanotubes (CNTs) are among the most avidly studied and utilized materials for multipurpose LIB electrode fabrication owing to their remarkable electronic conductivity, mechanical strength, resistance to chemical degradation, etc 22,23 . To the best of our knowledge, only little information can be found on the use of dry powdered CNTs along with a polymeric binder to directly press-coat electrode material onto a current collector in a completely solventfree approach. Therefore, the dry press-coating capability of the MWNT-PVDF composite powder was evaluated for the first time by measuring its adhesive and cohesive strength upon pressing. In addition, etched Al foil was selected as a new current collector ( Supplementary Fig. 1) to enhance adhesion by inducing an anchoring effect on the submicron pores of the foil surface. It was reported that a larger contact area with the Al2O3 passive layer improves the connection of the electrode film with the substrate surface 24-26 . As shown in the X-ray photoelectron spectroscopy (XPS) results ( Supplementary Fig. 2), the Al 2p photoelectron spectra indicate a higher amount of the Al2O3 layer on the etched Al foil than on the normal Al foil.
Herein, we developed a novel method to fabricate a solvent-free LiNi0.7Co0.1Mn0.2O2 (NCM712) electrode, namely, a dry press-coated electrode (DPCE), via a facile one-step hotpressing of premixed NCM712, multiwalled carbon nanotubes (MWNTs), and a dry PVDF powder mixture onto etched Al foil ( Fig. 1 and Supplementary Fig. 3). Additionally, the influences of the MWNT and binder content on the electrode structure and electrochemical performance were also studied. The DPCE with the optimal composition was then compared with conventional slurry-coated electrodes (SCE) on various aspects, such as morphology and electrochemical performance. Furthermore, to manifest an excellent 3D conductive network, ultrahigh-loading DPCE pouch cells were also fabricated." 3) In page 11 line 231-236, the better cohesion in DPCE is not only due to the MWNT but also the higher density (better contact).
→ Many thanks for the reviewer's constructive comment. As the reviewer pointed out the better cohesion in DPCE is not only due to the MWNT but also the higher density (better contact) and 5) The work was not well presented. There were so many different electrodes. It is hard to track which one was from each figure.
→ We thank the reviewer for raising this concern and for giving necessary guidelines as to how we can improve the conciseness and clarity of our manuscript. Therefore, portion of the main body especially the part that deals with the DPCE optimization was moved to the supplementary information. In this way, the manuscript can be more condensed and focus more on the comparative analysis with the SCEs. In response to the reviewer's comment, a comprehensive structural modification of the manuscript was made and provided in the revised manuscript below:

[Revised manuscript]
"Finally, the optimization of the DPCE composition was carried out by comparatively evaluating the rate capability, cycle performance, electrochemical impedance spectra (EIS), and charge-discharge profiles of the three different DPCEs using a half-cell test (Supplementary Note 1 and Supplementary Fig. 9). It was found that DPCE 1505 composition (NCM712/MWNT/PVDF -80/15/5) exhibited the best performance among all, which confirms the significance of MWNT and PVDF ratio on the electrochemical performance of DPCEs.
Furthermore, using the optimal composition, DPCEs with various other active materials, such as NCM622, LCO, and LFP, were also fabricated, all of which exhibited excellent rate capabilities ( Supplementary Fig. 10), verifying the versatile nature of the dry press-coating process." and DPCE 0515 (f).
6) When comparing the DPCE and SCE electrodes, did they have same mass loading? Was the SCE electrode calendered?
→ Many thanks for the reviewer's valuable comment. In order to make a fair comparison between DPCE and SCE, the areal mass loadings of electrodes were made the same, and both the electrodes were calendered before use. It is known that the mass loading of electrode is an important factor that affects the mechanical property and electrochemical performance, fixing the areal mass loading is crucial when comparing the intrinsic properties between different electrode samples.
Consequently, in the original manuscript, information about the areal mass loadings was mentioned when comparing between DPCE and SCE as shown below. However, for the case of high loading SCEs, it was not possible to fabricate beyond certain mass loading nor calender via roll press due to the structural instability and delamination caused by the solvent evaporation during the drying process. Hence, the highest attainable mass loading SCE was compared with the high loading DPCE.

[Original manuscript]
"The electrochemical properties of the DPCE and SCE are displayed in Fig. 3. DPCE clearly displays a better rate capability than SCE at all current densities (Fig. 3a). A long-term cycling test was also performed using a half-cell at 1.0 C with a mass loading of both electrodes at 8-9 mg cm -2 (Fig. 3b). The DPCE demonstrated much better cycling stability with an initial capacity of 170 mAh g -1 (1.0 C) and capacity retention of 67% after 400 cycles along with stable coulombic efficiency. In contrast, the SCE delivered an initial capacity of only 159 mAh g -1 (1.0 C) with the much lower capacity retention of 35% after 400 cycles and unstable coulombic efficiency after 300 cycles. This result can be explained by post-mortem analysis of the coin cell after cycling, wherein DPCE retains its original structure with negligible cracks as opposed to SCE, which showed detachments with noticeable voids around the active materials after cycling ( Supplementary Fig. 14)." 7) The formulation for the SCE was not appropriate. The binder content was too low for a 15% conductive additive, which contributes to the high EIS.    Regards to the concern about the binder content being too low for a 15% conductive agent and its contribution to the high EIS result, we would like to point out that many reports including  → Many thanks for the reviewer's valuable comment. The 1 C-rate was defined as the current density necessary to fully charge/discharge the LIB in 1h. Please note that the main active material used in our work i.e., NCM712 had a specific capacity of 160 mAh g -1 at 1 C. We apologize that some of the specific capacities of the tested electrodes at 1 C were omitted in the original manuscript. Hence, the 1 C values were included and provided in the revised manuscript as below:

[Revised manuscript]
"The electrochemical properties of the DPCE and SCE are displayed in Fig. 3. DPCE clearly displays a better rate capability than SCE at all current densities (Fig. 3a). A long-term cycling test was also performed using a half-cell at 1.0 C with a mass loading of both electrodes at 8-9 mg cm -2 (Fig. 3b). The DPCE demonstrated much better cycling stability with an initial capacity of 170 mAh g -1 (1.0 C) and capacity retention of 67% after 400 cycles along with stable coulombic efficiency. In contrast, the SCE delivered an initial capacity of only 159 mAh g -1 (1.0 C) with the much lower capacity retention of 35% after 400 cycles and unstable coulombic efficiency after 300 cycles. This result can be explained by post-mortem analysis of the coin cell after cycling, wherein DPCE retains its original structure with negligible cracks as opposed to SCE, which showed detachments with noticeable voids around the active materials after cycling ( Supplementary Fig. 14)." 9) In Fig. 7, the electrodes seemed too thick? What's the relative density or porosity of those electrodes? The porosity seems very high.
→ We thank the reviewer for mentioning these very reasonable concerns. As the reviewer mentioned, the thickness of the high loading electrodes in Fig. 7 (of the original manuscript) are definitely higher than the conventional slurry coated electrodes and therefore, the porosity should be considered as a crucial factor in the electrode design. As such, the porosity (ε) of the high loading electrodes was calculated based on the following equation 1,2 :  The calculation result shows that porosity of the high loading electrodes varies from 25 to 54%, which indicates that the porosity increases with the increase in the areal mass loading of electrodes.
However, these values are well within the average NCM cathode porosity values (30-50%) 3,4 despite greater areal mass loading and thickness of the high loading electrodes, owing to the uniaxial pressing in the dry press-coating process that produces more compact and denser electrode structure.
→ Many thanks for the reviewer's valuable comment. We also agree that the expression of energy density is more accurate when calculated based on the entire mass of cell. Moreover, we believe that the aforementioned representation can give our result an edge over others. In addition, the nominal voltage throughout the cycling test was calculated, and the result showed that the nominal voltages of DPCE pouch cell with varying areal mass loadings were around 3.9 V as shown below.
Therefore, this value was used for the calculation of energy densities.
Consequently, we recalculated the specific energy and volumetric energy density values based on the weight and volume of the accrual cell (including the pouch cell package) and provided the information in Fig. 6e    sincere apologies for confusing the reviewer with a mistakenly written true density value of NCM, and we would also like to thank the reviewer for pointing this out and giving us the opportunity to correct our mistake. As the reviewer mentioned, the true density value of 4.7 g/cm 3 should have been used for NCM active material instead of 3.7 g/cm 3 in the previous response. We thus recalculated the porosity of the high loading electrodes based on the correct NCM true density value using the following equation 1,2 :